This invention relates to methods and apparatus for maximizing the power output of gas lasers.
The present invention has particular application to Helium-Neon gas lasers.
Gas lasers of the kind to which the present invention relates have a cavity located between two end mirrors, and one mirror is coated to transmit a portion of the laser beam through the mirror.
The cavity of a gas laser of this kind can be operated in a variety of oscillation modes.
Depending to some extent on the length of the cavity, there may be a number of longitudinal modes in oscillation between the mirrors.
In addition to the longitudinal modes of oscillation, transverse modes can be sustained simultaneously. These transverse modes are known as transverse electric and magnetic (TEM.sub.mn) modes because the fields are nearly normal to the cavity axis. The subscripts m and n specify the integral number of transverse nodal lines across the emerging beam.
The simplest mode, TEM.sub.00, has a flux density across the beam cross section which is approximately gaussian; and there are no phase changes across the beam, as there are in other modes.
For many applications it is desirable to operate the laser in only the TEM.sub.00 mode while avoiding the production of higher order transverse modes. The problem of eliminating non TEM.sub.00 modes is accentuated by increasing the length of the laser.
One way of avoiding higher order transverse modes in the lasers of the prior art was to restrict the higher order modes by limiting the aperture size of the bore. This caused a higher diffraction loss of the higher order modes than of the TEM.sub.00 mode with the result that a purer TEM.sub.00 mode output was obtained; but it also could reduce the power output of the laser.
An important object of the present invention is to suppress high order transverse modes in a way that does not depend upon restricting the high order modes by diffraction loss.
It is a related object to suppress the high order transverse modes by constructing the mirror transmission in a way that causes each TEM.sub.00 mode to burn large enough holes to leave insufficient energy in the cavity for continuous lasing of higher order modes in the saturated gain, equilibrium state of operation of the laser.
Another factor which can reduce the power output of a gas laser is longitudinal mode sweeping. Longitudinal mode sweeping can result from variations in the mirror spacing caused by thermal effects.
The problem of mode sweeping becomes more acute as the length of the laser gets smaller. For example, with HeliumNeon gas lasers having a length of four inches or less, there may be only one mode existing in the cavity. lf this mode sweeps in, lasing action is produced and the laser produces the power output desired. If, however, the mode sweeps out of the cavity, there can be a 100% dropoff of power.
Since variation in the mirror spacing with temperature changes is very difficult to completely eliminate, a reduction in power output, particularly with the lasers of relatively short cavity length, has been a problem in the prior art.
It is an important object of the present invention to reduce the longitudinal mode sweeping in a gas laser by broadening the gain curve to increase the number of modes that can lase in the cavity to thereby reduce the drop in power output resulting from mode sweeping (as compared to the drop in power output for an equivalent length laser having fewer modes under a more narrow gain curve).
It is a related object of the present invention to broaden the gain curve by using an isotope mixture of one of the gases in the cavity.
Another factor that has contributed to reducing the power output of Helium-Neon gas lasers has been unwanted infrared lasing activity.
Because the 0.6328 micron red photon transition shares the same upper level state as a 3.39 micron infrared photon transition, both transitions compete for the resources of the same, common source. If the superradiance of the infrared transition is not suppressed, the power output of the 0.6328 micron transition can be substantially reduced. This problem became particularly severe with increasing gain in Helium-Neon lasers. At about 10 milliwatt power or greater output the 3.39 micron transition can come strongly into play so that a lot of power may be absorbed by the 3.39 micron infrared transition in Helium-Neon lasers designed for power outputs of 10 milliwatt or more for the 0.6328 micron.
In the prior art magnets were positioned near the laser gain tube for the purpose of suppressing the 3.39 micron transition by means of Zeeman splitting. The magnetic field acted on the Neon gas and caused Zeeman splitting and broadened the transition at 3.39 microns.
The use of magnets to suppress the 3.39 micron transition is difficult to apply to a laser having a coaxial structure because it is difficult to put the magnets close to the bore in a coaxial laser. The magnets must be very large, to be effective, if mounted on the outside tube of the coaxial laser.
It is an important object of the present invention to suppress the 3.39 micron transition in a Helium-Neon gas laser without the need for magnets.
It is a related object to reduce the peak gain in the cavity by an amount which highly suppresses the superradiance of the 3.39 micron transition and to use an isotope mixuture of the Neon gas for providing this reduction in the peak gain.
It is another related object to construct a mirror transmission to be an amount low enough to cause the gain of the 0.6328 micron red photon transition to assist substantially in the suppression of the gain of the 3.39 micron infrared photon transition.
Another problem in obtaining smooth power output from Helium-Neon gas lasers is the problem of reducing beating noise resulting from frequency offset due to dispersion.
It is an important object of the present invention to reduce the beating noise by increasing the number of gain curves and to use a mixture of isotopes for the Neon gas for this purpose.